String theory is about the idea that reality is best modeled as, notionally, strings - i.e. the mathematics looks a bit like them if you sort of squint and look at it really hard.
But there's a really good reason strings are a compelling here: because near as we can tell, vibrational frequency is really important to particle physics - the energy of a photon is quantized as E=nhf , (Energy = integer * planck's constant * frequency). And this seems to scale all the way down - it works for photons, it works for electron orbitals, and it comes into play dealing with the virtual particles which are exchanged in force interactions.
So however you describe reality, whatever you do, your abstraction pretty assuredly needs to somehow recover the vibrational importance, which implies certain kinds of "geometry". You get "strings" because strings are one of the most fundamental structures which can hold a vibrational mode, and importantly, they kind of neatly explain the quantization as standing waves - closed strings can only setup a standing wave at certain frequencies depending on their length, same with open strings (the wave needs to be zero at the ends of the "string" or it'll lose energy).
Of course this is all my very ad hoc handwavey explanation of it, but the point is, there's a lot of very basic physics which works quite well in the model. It did not, and does not, just come from nowhere even at this very high level.